1. Field of the Invention
The present invention relates to a method of reducing a noise generated from a speaker at the time of power on/off and reset of an electronic equipment.
2. Description of the Related Art
Conventionally, in the case of driving a speaker in an electronic equipment such as an audio output device, a digital signal is inputted into a D/A converter, an analog output from the D/A converter is inputted to an Op-amp, an output from the Op-amp is outputted to an output terminal, and then, a sound or the like is outputted from a speaker via an external capacitor.
However, in such a case where a signal of ½VDD (signal ground) is inputted as an input signal of a D/A converter, an output of the D/A converter sharply rises up to ½VDD level at the time of power on, and an output of an Op-amp also shows a sharp change from a ground (GND) level or a high impedance state to a signal ground level. Therefore, the sharp change in potential is inputted to a speaker via a capacitor, which causes the noise. Also, in such a case where the output of the Op-amp shows a sharp change from the signal ground potential to the GND potential or a high impedance state at the time of power off or the reset, the sharp change in the potential is inputted to the speaker via a capacitor and causes the noise therein.